An immunoassay for .beta.-endorphin including the use of various radiolabelled .beta.-endorphin tracer compounds is described in U.S. Pat. No. 4,096,237, issued June 20, 1978, inventor, C. H. Li. Tritiated .beta.-endorphin is disclosed in such patent.
The synthesis of N.sup..alpha. -Boc-3,5-diiodotyrosine-(OBrBzl) is described by Lemaire et al., J. Am. Chem. Soc. 99, 1577 (1977).
Tritiation of a 3,5-diiodotyrosine ACTH analog is disclosed by Ramachandran and Behrens, Biochem. Biophys. Acta 496, 321 (1977).